Aluminum hydroxide, also known as alumina trihydrate, is the precursor of many alumina-based products, including calcined alumina used for making metallic aluminum by reduction. Aluminum hydroxide is most commonly obtained from alumina-containing ores, such as bauxite. Recovery of the alumina content of bauxite is generally accomplished by the well-known Bayer process which involves the digestion of the bauxite with a caustic medium at elevated temperatures and pressures. Digestion of the bauxite results in a saturated sodium aluminate liquor, commonly referred to as “pregnant liquor” from which the alumina content is recovered by precipitation, usually through addition of seed aluminum hydroxide.
In the Bayer process for the production of alumina, bauxite ore is pulverized, slurried in water, and then digested with caustic soda, also known as sodium hydroxide, at elevated temperatures and pressures. The caustic solution dissolves oxides of aluminum, forming an aqueous sodium aluminate solution. The caustic-insoluble constituents of bauxite ore, referred to as “red mud”, are then separated from the aqueous phase containing the dissolved sodium aluminate. Solid aluminum hydroxide is precipitated out of the solution and collected as product.
In more detail, the pulverized bauxite ore is fed to a slurry mixer where a water slurry is prepared. The slurry makeup water is typically spent liquor (described below) and added caustic soda. This bauxite ore slurry is then diluted and passed through a digester or a series of digesters where, under high pressure and temperature, about 98% of the total available alumina is released from the ore as caustic-soluble sodium aluminate. After digestion, the slurry passes through several flash tanks wherein the pressure of the digested slurry is reduced from several atmospheres to one atmosphere and the temperature of the slurry is reduced from about 200° C. to about 105° C.
The aluminate liquor leaving the flashing operation contains from about 1 to about 20 weight percent solids, consisting of the insoluble residue that remains after, or is precipitated during, digestion. The coarser solid particles may be removed from the aluminate liquor with “sand trap” cyclones. The finer solid particles are generally separated from the liquor first by settling and then by filtration, if necessary. The slurry of aluminate liquor leaving the flash tanks is diluted by a stream of recycled washer overflow liquor. Any Bayer process slurry taken from the digesters through any subsequent dilution of the slurry, including the flash tanks, but before the primary settler, is referred hereinafter as the primary settler feed.
Normally, the primary settler feed is thereafter fed to the center well of the primary settler, where it is treated with a flocculant. As the mud settles, clarified sodium aluminate solution, referred to as “green” or “pregnant” liquor, overflows a weir at the top of the primary settler and is collected. This overflow from the primary settling tank is passed to the subsequent process steps.
The clarity of the primary settler overflow is crucial to the efficient processing of aluminum hydroxide. If the aluminate liquor overflowing the settler contains an unacceptable concentration of suspended solids (at times from about 10 to about 500 mg of suspended solids per liter), it must be further clarified by filtration to give a filtrate with no more than about 10 mg suspended solids per liter of liquor. The treatment of the liquor collected after the primary settlement to remove any residual suspended solids before aluminum hydroxide is recovered is referred to as a secondary clarification stage.
The clarified sodium aluminate liquor is seeded with aluminum hydroxide crystals to induce precipitation of alumina in the form of aluminum hydroxide, Al(OH)3. The aluminum hydroxide particles or crystals are then separated from the concentrated caustic liquor, and the remaining liquid phase, the spent liquor, is returned to the initial digestion step and employed as a digestant after reconstitution with caustic.
Bauxite is found in many parts of the world and the composition of the ores may vary from place to place. Many bauxites contain organic carbon (also referred to as “organic impurities”) that will be co-extracted with the alumina content of the ore during digestion and will contaminate the produced liquor. Most of the organic carbon content found in the ores consists of high molecular weight compounds, a portion of which will decompose to lower molecular weight compounds during the caustic digestion process, thereby producing a whole spectrum of organic salts dissolved in the liquor. Since the Bayer process involves extensive recycling of the used caustic liquor to the digestion stage, the organic carbon content of the liquor will continuously increase, reaching levels ranging from about 5 grams carbon/liter of liquor to about 40 grams carbon/liter of liquor depending on the type of bauxite being processed. The accumulation of organic carbon content can reach such high levels so as to seriously interfere with the economic and efficient production of aluminum hydroxide with an organic carbon content low enough so that the aluminum hydroxide can be used for applications requiring a low total organic carbon content.
Since the control of organic carbon levels in Bayer process liquors is an important facet in the production of aluminum hydroxide, several methods have already been developed for such organic carbon level control. It has been suggested in U.S. Pat. No. 4,046,855 (Schepers et al.) that organic impurities can be removed from Bayer process liquors by contacting the liquor with a magnesium compound which will form a precipitated mixture of magnesium and aluminum hydroxides. This precipitate, according to the patent, can remove some of the organic impurities either by adsorption or by chemisorption. The magnesium compound may be added at any stage of the Bayer process, additions prior to digestion or to the digested slurry are preferred. Although this process is capable of removing at least a portion of the organic impurities, the formation of a precipitated hydroxide mixture creates operational difficulties. On the one hand, the precipitated hydroxide mixture will contain aluminum hydroxide and this results in product alumina loss; on the other hand, the precipitated mixture has to be separated from the rest of the treated liquor and this involves additional processing steps and/or a definite increase in the quantity of the total mud load which requires disposal.
In U.S. Pat. No. 4,101,629 (Mercier et al.), a barium-containing compound is added to Bayer process liquors. The barium compound precipitates as barium aluminate and the precipitated material may also include barium salts of organic impurities present in the liquor. As in the previously discussed patent, this process involves precipitation of a compound which has to be removed from the treated liquor requiring settling and/or filtration equipment and additional processing steps. The process allows recovery and reuse of the filtered barium compound by calcination; however, the well-known toxicity of barium salts may create an unacceptable environmental and/or health risk not justifiable by the purification results obtainable by it.
In U.S. Pat. No. 4,335,082 (Matyasi et al.), organic impurities are removed from impure Bayer liquors by caustifying the liquor with lime, followed by evaporation of the causticized liquor. Evaporation will result in the precipitation of solids containing a large quantity of the organic impurities from the liquor. The solids are separated and then discarded. This method assures the removal of satisfactory quantities of organic impurities from the liquor, but the problems associated with the process render it impractical and expensive. To achieve good purification, large volumes of liquor have to be treated with lime and evaporated. These liquor treatment processes involve the use of large quantities of lime and extensive energy input and large soda value losses. “Soda value” refers to any sodium salt found in Bayer process liquor. Specifically, soda associated with sodium aluminate, free sodium hydroxide and sodium carbonate. All of these are derived from a key raw material, caustic soda, which represents a major raw material cost. Therefore, all Bayer process refineries operate to minimize the loss of soda value.
A similar purification process is disclosed in U.S. Pat. No. 4,280,987 (Yamada et al.). In this process, Bayer liquor is first evaporated, then calcined at high temperature after its alumina and caustic content is adjusted to a predetermined level. This process, known in the Bayer industry as “liquor burning,” is an effective means of organic impurity removal. Its disadvantages are associated with the large volumes to be evaporated and then calcined, which require substantial capital and energy expenditures. Liquor burning also requires air emission control equipment in order to avoid air pollution issues.
In U.S. Pat. No. 4,215,094 (Inao et al.), a copper-catalyzed wet oxidation process is recommended for the oxidation of organic impurities, followed by addition of a sulfur-containing compound to remove the copper catalyst as a precipitate. The oxidation is accomplished under elevated temperature and pressure conditions in the presence of a catalyst and molecular oxygen. This process has several disadvantages in that a high temperature-pressure digestion has to be applied which involves the use of expensive pressure vessels and substantial energy usage. In addition, the copper catalyst has to be removed from the treated liquor to avoid contamination. Disposal of the removed copper sulfide can create environmental and/or health hazards. Similarly, in U.S. Pat. No. 4,663,133 (Malito et al.), organic impurities are oxidized at elevated temperature and pressure by feeding molecular oxygen directly into the bauxite digestion vessels. The amount of oxygen used is limited to below the solubility of oxygen in the liquor and is sufficient to destroy only a portion of the organic impurities. Moreover, there is the potential of explosion due to the high pressures required.
In Japanese Patent No. 53-146,259 (Kazama et al.) various oxidizing agents, such as sodium peroxide powder and 50% hydrogen peroxide, are used to destroy part of the organic impurities. Though effective, these reagents are expensive and hazardous. Also, the color of pregnant Bayer liquor is removed by passing a small stream of air containing 1% ozone through pregnant Bayer liquor. However, this dilute ozone stream only removes color and is not sufficient to oxidize any of the organic impurities.
It would be desirable to provide an aluminum hydroxide, made via the Bayer process, with lower levels of total organic carbon. It also would be desirable if the process used to provide an aluminum hydroxide, made via the Bayer process, with lower levels of total organic carbon, did not require high temperature and pressure and is, relatively speaking, environmentally benign.